Could you try please using <dns enable="no"/> for the default network (and any other networks you may have)?
sudo virsh net-edit default
Then try:
sudo apt purge dnsmasq
Maybe that way we can completely get rid of dnsmasq?
Or libvirt will still crash due to:
This would initially break DNS inside any non-Whonix VMs. That could be fixed by using a public DNS resolver.